Abstract: The internal boundary in flow field, which must be designated in the cylindrical polar coodinate system, can be avoided by using body-fitted coordinate system (BFC). Consequently, computed flow of molten steel and paths of alloying additions are able to cross smoothly the geometric centerline of cylindrical verssel.Motion of particles is calculated in three dimensional coordinate system, the modification of parameters,such as the drag coefficient and density in the two phase region, is examined. Some factors affecting the trajectories of alloying additions are discussed.